The present invention relates to an ion detector for a Time of Flight mass spectrometer, a Time of Flight mass analyser, a mass spectrometer, a method of detecting ions and a method of mass spectrometry.
Time of Flight mass spectrometers comprising an ion detector coupled to a one bit Time to Digital Converter (“TDC”) are well known. Signals resulting from ions arriving at the ion detector which satisfy defined detection criteria are recorded as single binary values associated at a particular arrival time relative to a trigger event.
It is known to use a fixed amplitude threshold to trigger recording of an ion arrival event. Ion arrivals recorded for subsequent trigger events are added to a histogram of events which is then presented as a spectrum for further processing. TDCs allow efficient detection of weak signals where the probability of multiple ions arriving in close temporal proximity is relatively low. However, once an ion event has been recorded then there is a significant time interval (“dead time”) following the event during which time no further events may be recorded.
A disadvantage of the known ion detector with a one bit TDC detector is its inability to distinguish between a signal arising from the arrival of a single ion and a signal arising from the arrival of multiple ions at the same time since the resulting signal only crosses the threshold once irrespective of whether a single ion arrives or multiple ions arrive. As a result, both of these situations result in only one event being recorded.
At high signal intensities the problem of being unable to discriminate between a single ion arrival event and multiple ions arriving, together with the problem of dead time effects results in some ion arrival events not being recorded or the actual number of ions being incorrectly recorded. This results in an inaccurate representation of the signal intensity and also results in an inaccurate measurement of the arrival time. These effects place an effective limit on the dynamic range of the detector system.
More recent commercial Time of Flight mass spectrometers have moved away from using TDC detector systems and utilise instead an Analogue to Digital Converter (“ADC”) based detector system.
ADCs operate by digitising a signal output from an ion detector relative to a trigger event. The digitized signal from subsequent trigger events may be summed or averaged to produce a spectrum for further processing. State of the art signal averagers are capable of digitizing the output of detector electronics at 4 or 6 GHz with eight, ten or twelve bit intensity resolution.
Using an ADC detector advantageously allows multiple ion arrivals to be recorded at relatively high signal intensities without the detector suffering from distortion.
Whilst current state of the art ADC detector systems have several advantages over earlier TDC detector systems, ADC detector systems suffer from the problem that detection of low intensity signals is generally limited by electronic noise from the digitiser electronics, detector and amplifier used. This effect limits the dynamic range of ADC detection systems. Another disadvantage of a conventional ADC detector compared with a TDC detector is that the analogue width of the signal generated by a single ion adds to the width of the ion arrival envelope for a particular mass to charge ratio value in the final spectrum.
The ability of a mass spectrometer to detect a low level species in the presence of or close proximity of another species at high level is known as the abundance sensitivity. Abundance sensitivity may be defined as the ratio of the maximum ion current recorded at a mass m to the ion current arising from the same species recorded at an adjacent mass (m+1).
Single channel ADC systems have limited abundance sensitivity because mismatch of the high frequency detector impedance causes ringing after a large ion signal. The level and duration of the ringing obscures low level signals arriving after a large peak and so low level ion signals can go undetected.
FIG. 1A shows an ion signal having a λ of 10 (wherein λ corresponds with the number of ions per push per peak). FIG. 1B shows an artifact which is typically observed in an ADC detector system following the arrival of an intense ion beam. The artifact is a time delayed image of the signal. FIG. 1C shows how a threshold set at λ equal to 1 can discriminate between a real small signal and an artifact of a large signal having a λ of 10. FIG. 1D illustrates a problem with current state of the art ADC detector systems. The threshold is set at λ equal to 1 and is effective in discriminating between a real small signal and an artifact of a large signal having a λ of 10. However, the threshold is not able to discriminate an artifact of a very large signal having a λ of 20.
As will therefore be readily appreciated by those skilled in the art, current commercial Time of Flight mass spectrometers employing ADC ion detectors suffer from the problem of having a limited abundance sensitivity. Consideration has therefore been given as to how to improve the abundance sensitivity of commercial Time of Flight mass analysers.
One attempt at improving the abundance sensitivity of a Time of Flight mass analyser is to revert to using a TDC based detector system. According to a known arrangement a double or chevron Micro Channel Plate (“MCP”) ion detector may be used to detect ions and convert the ions to electrons. The electrons are then detected using multiple metal anodes each of which is connected to an individual TDC. The use of multiple anodes reduces the problem of deadtime effects and the Inability to distinguish between multiple ions arriving at substantially the same time and a single ion arrival event since multiple ions arriving at substantially the same time are likely to be detected by different anodes.
The known approach using TDCs and multiple anodes effectively comprises a multiple pixel detection scheme which splits an ion signal into many channels. It is important that an individual ion strike should ultimately illuminate only a single pixel on the detector to take advantage of the increase in dynamic range that multiple detector channels afford. A double or chevron MCP arrangement is used because it retains the spatial information of the original ion strike with little signal flaring such that the output electron cloud only illuminates a single pixel or anode. Additionally, in a chevron configuration, the double or chevron MCP has enough gain to be amenable to simple amplification that can then trigger a threshold in a TDC system. Splitting the signal into many channels ensures that each anode receives a lower average ion count and a low level signal can be detected without interference from a high level signal thereby improving the abundance sensitivity characteristic.
However, despite certain advantages in using a detector arrangement comprising a double MCP, multiple anodes and multiple TDCs, such an arrangement remains only effective at detecting an ion signal at relatively low or moderate ion intensities.
As will be appreciated by those skilled in the art, ion sources are being developed which are becoming increasingly brighter and state of the art and future ion detectors need to be able to operate at high ion currents. However, the known multiple anode and multiple TDC ion detector arrangement is unable to provide sufficient gain for the detector electronics to function at high ion currents (i.e. >107 events/second). Furthermore, the known detector arrangement also suffers from the problem of crosstalk between the metallic anodes which degrades the performance of the ion detector.
ADC based ion detector systems are also unable to operate with very bright ion sources i.e. >107 events/second. Furthermore, ADC detector systems suffer from the problem of limited abundance sensitivity due to the effects of ringing after a large ion signal as discussed above.
It is therefore desired to provide an improved detector system for a Time of Flight mass spectrometer which is capable of processing e.g. 109 events/second and which does not suffer from the problems inherent with both known ADC and TDC detector systems.